Circuit breaker having pressurized liquified gas continuously maintained above instantaneous vapor pressure

ABSTRACT

A circuit breaker having a circuit interrupting chamber containing a liquefied dielectric gas with a fixed contact member disposed so as to lie in the liquefied gas and a movable contact member disposed in the chamber so as to have its contact making surface within the liquefied gas for all of its positions relative to the fixed contact, and having means, such as a hydraulic accumulator or a compressed gas compartment communicating with the circuit interrupting chamber; i.e., other than the gaseous phase of the liquefied dielectric gas for continuously maintaining the liquid pressure above the critical pressure of the dielectric gas.

United States Patent [72] lnventor Jean Louis Gratzmuller [56]References Cited NeuilIy-sur-Seinc, Hauts-de-Seine, France UNITED STATESPATENTS [21] PP'- 867,959 3,150,245 9/1964 Leeds et al 200/14s .7 [22]Filed Oct. 20, 1969 FOREIGN PATENTS [45] Patented Mar. 9, 1971 [32]Priority Jan.2l, 1965 1,281,324 12/1961 France 200/1508 [33] France1,323,669 3/1963 France 200/1508 [31] 2 1,349,314 12/1963 France200/1508 Continuation application sen 1,143,890 2/1963 Germany....200/150 521,852, Jan. 20, 1966, abandoned, 1,164,536 3/1964 GermanvmZOO/148.5

609,589 10/1948 Great Britain ZOO/148.7

Primary Examiner-Robert S. Macon Attorney-Jacobi, Davidson, Lilling &Siegel ABSTRACT: A circuit breaker having a circuit interrupting chambercontaining a liquefied dielectric gas with a fixed con [54] PSRESSURIZEDtact member disposed so as to lie in the liquefied gas and aINSTANTANEOUSUVAU S E E movable contact member disposed in the chamberso as to 11 9 D PDR R s URE have its contact making surface within theliquefied gas for all of its positions relative to the fixed contact,and having means, [52] US. Cl. 200/148, such as a hydraulic accumulatoror a compressed gas compart- 200/150 ment communicating with the circuitinterrupting chamber; [51] Int. Cl. H0lh 33/68 i.e., other than thegaseous phase of the liquefied dielectric gas [50] Field of SearchZOO/148.2, for continuously maintaining the liquid pressure above the148.7, 148, 150 (foreign), 150.1, 150 (G), 148.1

critical pressure of the dielectric gas.

PATENTEU MAR 919m sum 1 a; 3

PATENTEDNAR 91971 SHEET 2 U? 3 FIGS FIGS

PATENTED MAR 9 I971 SHEET 3 OF 3 a15:5 :E .i

FIGS

CIRCUIT BREAKER HAVING PRESSURIZED LIQUEFIED GAS CONTINUOUSLY MAINTAINEDABOVE INSTANTANEOUS VAPOR PRESSURE CROSS REFERENCE TO RELATEDAPPLICATIONS This application is a continuation of application Ser. No.521,852, filed Jan. 20, 1966, now abandoned.

FIELD OF INVENTION This invention is for improvements in or relating toelectric circuit breakers.

On the basis of the insulating fluid in which the circuit is broken, theknown kinds of circuit breakers can be divided into two main kinds withrupturing properties compatible with the ever increasing short circuitpowers of highand mediumvoltage networks. These two kinds are:

1. oil circuit breakers containing either a large volume of oil, as inUS. practice, or a reduced volume of oil as in European practice, and

2. gas circuit breakers such as air blast circuit breakers and morerecently sulfur hexafluoride (SF6) circuit breakers.

This invention relates to circuit breakers using a novel dielectricagent for arc extinction.

The invention relates to a circuit breaker wherein the insulating fluidin which the arc is quenched is a liquefiable dielectric gas keptliquid, at least in the circuit breaker rupture chamber, at normaloutside ambient temperatures by pressurization, the fluid being chosenamongst those having, when in the gas state and liquid state, physical,chemical and/or electrical properties better than air and oil,respectively, for are quenching.

The only liquid insulants so far used in circuit breakers i.e., oils andsimilar products -have the advantage inherent in their liquid state,including a higher heat transfer coefficient than gases and therefore agreater rate of heat dissipation of the arc column, and therefore ashort time constant which helps to break AC arcs as the voltage ispassing through zero because of the rapidity with which the dielectricmedium is restored. However, a disadvantage of oil is that the arcdecomposes it, conductive carbonaceous products being formed.

In the case of gas circuit breakers, endeavors have been going on for anumber of years to replace air by a gas having better properties (timeconstant, dielectric strength, rupturing capacity) than air, leading tothe construction of SF circuit breakers. As well as having advantagesover air, SF has the advantage over oil that arcing does not produce anyconductive decomposition product. Also, most of the decompositionproducts recombine immediately to form SF However, a disadvantage of SFcircuit breakers is that they must have some form of heating to keep thesulfur hexafluoride in the gas state during cold weather; SP circuitbreakers are therefore dependent upon external power and notsubstantially independent in the same way as oil circuit breakers.

The circuit breakers according to the invention combine the advantagesof oil circuit breakers and gas circuit breakers but are free from thedisadvantage of both kinds.

According to the invention, the new circuit breakers are of very similarconstruction to small-oil-volume circuit breakers, but the oil isreplaced by a pressurized liquefied gas such as sulfur hexafluoride SFselenium hexafluoride, SeF C F and CF;, SP gases, electronegative gases,and perhalogenated hydrocarbons, such as CCl F known under the name of Freons, and so on. These gases will be referred to hereinafter by thegeneral term of liquefiable dielectric gases.

SUMMARY OF THE INVENTION In a circuit breaker according to theinvention, the liquefiable dielectric gas is kept liquid at asubstantially constant pressure which for some gases can be near orabove the critical pressure of the gas. As a rule, the pressurizingmeans are such that the pressure variations produced by temperaturevariations are of limited amplitude.

In circuit breakers of this kind, one advantageous way of pressurizingthe fluid sufficiently to maintain the same permanently liquid is to usea known hydraulic accumulator of the spring or compressed gas kind andwith or without a separating piston. The circuit breaker need not thenbe dependent upon an external source of power in normal operation. Moreparticularly, no heating or cooling means are required for the circuitbreaker to operate outside in all the extreme temperature conditionsfound, for instance, in continental France of from -40 to +60 C.

A circuit breaker according to the invention can comprise a hermeticrupture chamber in which at least one fixed contact and one movingcontact are disposed and which is tilled with a liquefiable dielectricgas in the liquid state. A circuit breaker according to the inventioncan also comprise a hydraulic accumulator whose liquid compartment isfilled with the same gas in the liquid state and is hydraulicallyconnected to the rupture chamber and experiences the pressure applied bythe resilient means at the accumulator, such pressure being enough tokeep the gas liquid in normal outside temperature conditions.

According to the invention, connecting means between the rupturechamber, the accumulator and/or the outside are provided to enable airto be bled from the system and taking into operation and to deal withvolume variation caused by temperature variations.

The invention will be more clearly understood from the followingdetailed description and from the accompanying drawings which showvarious embodiments of the invention as nonlimitative examples and inwhich:

BRIEF DESCRIPTION OF THE DRAWING VIEWS FIG. 1 is a diagrammaticsectioned view of a circuit breaker according to the invention;

FIG. 2 shows a preferred embodiment of the invention;

FIG. 3 is a separate view of a spring-biased hydraulic accumulator forpressuring the system;

FIG. 4 is a diagrammatic sectioned view of a circuit breaker accordingto the invention;

FIGS. 57 each show a way of filling the circuit breaker with liquefiablegas;

FIG. 8 shows a level-indicating system for a circuit breaker accordingto the invention, and

FIG. 9 shows another embodiment of a circuit breaker according to theinvention wherein the liquid dielectric is pressurized by a pocket ofgas disposed at the top of the rupture chamber.

DETAILED DESCRIPTION FIG. 1 shows by way of example a ground-mountedcircuit breaker having a rupture chamber 2 at the top of an insulatingcolumn 4. In construction the circuit breaker resembles thesmall-oil-volume type circuit breakers. A movable contact 6 can move inthe chamber 2 and is borne on by rubbing contacts 8 connected to a part10 of a line to be broken and which can engage in a fixed contact ortulip contact 12 connected to another part 14 of such line. The movingcontact 6 is operated for closing and tripping by a conventionalmechanism 16 including for instance, a hydraulic jack to which thecontact 6 is connected by an insulated rod 18.

In the embodiment shown in FIG. 1, the internal volume of the chamber 2is closed hermetically; more particularly, a gland 20 is provided wherethe moving contact 6 passes through the chamber end 22. An insulatedtube 24 extends into the internal volume or space 19 of the chamber 2,goes through the column 4 and terminates at a liquid compartment 26 of ahydraulic accumulator 28 which is at earth potential. The liquidcompartment 26, which can be bounded by a free piston 30, is keptpressurized by the conventional resilient means of the accumulator, suchas a pressurized gas cushion 32 trapped in the second compartment of theaccumulator. The resilient means can take the form of a spring 33, asshown in FIG. 3.

The internal volume or spaces of the chamber 2, tube 24 and compartment26 are filled completely by a liquefiable dielectric gas, such as SF orFreon, which is kept liquid by the pressure applied by the gas cushion32. Of course, the gas used for the cushion, e. g. nitrogen, has aliquefaction pressure appreciably greater than the liquefaction pressureof the dielectric gas used. A conventional valve 34 is provided forreinflatingthe accumulator, and a pressure gauge 36 gives a check on thepressure of the dielectric fluid in the circuit breaker and helps toestimate the quantity of liquid contained in the compartment 26; thepressure gauge 36 can, if required, control safety or pressure-restoringoperations automatically. An escape valve 38 at the top of the rupturechamber 2 is used to bleed air from the system when the same is takeninto 1 operation.

In a ground-mounted circuit breaker of the kind just described, theusual practice is for the column 4 to be filled with insulating oil, asis normal practice with known circuit breakers. However, the gland 20may be difficult to keep tight after a time because of erosions of themoving contact 6 by the arc at rupture. In another embodiment of theinvention, therefore, which is shown in FIG. 2, the hermetic separation,provided by the gland 20 and the chamber end 22, between the rupturechamber 2 and the interior of the column 4 is omitted and all thesespaces are filled with pressurized liquefied dielectric gas. Only oneseal between a stationary element and a moving element then remains andit is provided by a gland 40 at the bottom of the column 4, the gland 40cooperating with jack rod 42 which can be completely smooth and does notexperience erosion.

Another feature of the embodiment shown in FIG. 2 is the provision ofbleeding of a kind, making it unnecessary to operate the safety valve38, the same not being readily accessible and being in the live part ofthe circuit breaker when the same is in operation. Accordingly, anescape tube 44 is disposed at the top of the rupture chamber 2 andcomprises a nonreturn valve 46 through which fluid can flow only in thedirection indicated by the arrow 48. The tube or duct 44 is insulatedand forms a return to the liquid compartment 26 of the accumulator 28.Similarly, the duct 24 connecting the liquid compartment 26 to theinterior of the column 4 and chamber 2 has a nonreturn valve 50 throughwhich fluid can flow only from the accumulator towards the circuitbreaker.

A draincock 52 which is disposed at the bottom of the duct 44 and whichis therefore at earth potential and readily accessible, can connect theinterior of the circuit breaker to atmosphere for air bleeding purposesat startup. The accumulator 28' can be isolated by a stop cock 54 duringthis operation.

Instead of the stop clock 54, a simple nonreturn valve can be used, inwhich event there is no need to open the stop cock 54 at startup. Ofcourse, these two valve devices 52, 54 can be combined as a singlethree-way valve. The duct 24' has fitted to it a pressure gauge 36 and abranch 56 which can be connected to a source of liquefiable dielectricfluid to fill or topup the system. A safety valve 57 can be providedsomewhere in the system. Since most liquefiable dielectric gases aremuch heavier than air, opening the drain cock 52 at the start of fillingensures removal of all the air in the system.

Another variant shown in FIG. 2 is the accumulator 28 since the liquidcompartment 26 and the gas compartment 32 are not separated from oneanother by moving pistons; instead, the resilient cushion 32 ofcompressed gas, for instance, nitrogen, acts directly on the liquifieddielectric gas contained in the compartment 26. The circuit justdescribed which connects the interior of the circuit breaker to theaccumulator and selectively to atmosphere and a dielectric fluid source,also provide a one-way flow of the fluid in closed circuit and automaticbleeding of foreign gases.

The accumulator is charged to a pressure P (or else, in the case shownin FlG. 3, the accumulator spring applies a pressure P to the piston)such that the liquefiable dielectric gas is kept liquid at normalambient temperatures. For instance, if the liquefiable dielectric gas issulfur hexafluoride, SF the accumulator can be pressurized to at least35 kg/cm.2 in normal temperature conditions. For Freons the pressure canbe merely about from 15 to 20 kg/cmF. lf the temperature drops, thevolume of liquefiable dielectric gas in the circuit breaker decreasesand the nonreturn valve 46 stays closed, but the valve 50 opens and acorresponding volume of liquefiable dielectric gas is transferred fromthe liquid chamber 26 of the accumulator to the circuit breaker; Ofcourse, the volume of the gas compartment 32 increases correspondinglyand the pressure decreases, but since the temperature has dropped thedielectric fluid remains in the liquid state. In the event of atemperature rise, there is a transfer from the circuit breaker to theaccumulator through the duct 44, in the direction indicated by the arrow48. The transfer is accompanied by a general pressure increasethroughout the system, the dielectric fluid therefore tending to remainin the liquid state despite the temperature rise. Clearly, therefore,there is a slow closedcircuit flow of the dielectric fluid in accordancewith tempera ture variations. Because of this flow, the liquid can befiltered and the gases return to the accumulator. This leads to acontinuous and automatic purging or deaeration of the system.

With some liquefiable dielectric gases having a critical temperatureabout 45 C. in the case of SP near the temperature to which some circuitbreaker parts may rise when exposed to the sun, the pressure can readilygo above the critical pressure; indeed, it may be advantageous for thepressure to be kept permanently above the critical level to reliablyprevent volume variations causing changes of state. Advantageously, inall cases, accumulator volume is so adapted to the volume of the circuitbreaker containing the liquefied dielectric that pressure variationscaused by temperature variations always maintain the dielectric in theliquid state, at least in that part of the circuit breaker where thearcis produced.

Although some Freon gases have dielectric properties worse than thedielectric properties of SF the use of all or some Freons" in circuitbreakers according to the invention may be advantageous, since theirfreezing temperatures are very low and their critical temperatures areconsiderably above all the outside temperatures met with in practice.For the rest, they have a relatively low liquefaction pressure and,depending upon the particular kind actually used, the system can operateat pressures of from 15 to 2'0 kg/cm The circuit breaker shown in FIG. 4is similar to the circuit breaker shown in FIG. 1 in that the interior19 of the rupture chamber 2 is filled with a liquefied dielectric gas,such as SF and is separated, by the chamber end 22 and a gland 20, fromthe interior of the insulated column 4 which can be filled withinsulating oil. The circuit breaker also resembles the circuit breakershown in FIG. 2 to the extent that it comprises a closed circuit for theflow of liquid dielectric between the accumulator 28 and the rupturechamber 2. The closed circuit comprises a duct 44 which extends at thetop into the rupture chamber 2 and which communicateswith a duct 44connected to the accumulator 28, a filter'60 being provided between theducts 44 and 44', stop cocks 62, 54 being provided one each on each sideof the filter 60. The closed circuit also comprises ducts 24, 24connected to the bottom of the chamber 2 and to the accumulator 28,respectively. As already stated, temperature variations produce a flowof the dielectric, the flow being possible only in the directionindicated by the arrows beside the ducts 44 and 24 (since the nonreturnvalves 46, 50 are provided), so that continuous filtering of thedielectric is provided. To replace the filter, the two stop cocks 54, 62are closed whereafter the filter element can be changed. If thetemperature differences which the system experiences are small or if itis required to speed up filtering, a circulating pump 64 can be providedfor the closed circuit.

It is advantageous to filter the SP or similar substance through thefilter 60 in operation but it is also advantageous to make the firstfill with a fluid thoroughly freed from foreign substances which may bepresent, for instance, in the cylinders in which the liquefied gas issupplied. Advantageously, there fore, a circuit breaker according to theinvention is filled for the first time by fractionated distillation, forinstance, by one of the procedures shown in FIGS. 5 and 6. Preferably,in order not to waste SF nitrogen is injected from a cylinder connectedto the branch 56, to remove all the air from the system before fillingbegins. Nitrogen is relatively cheap and, even if it does stay in thecircuit, will not cause disturbances, particularly if the accumulator 28is of the kind not having a separating piston. Once the air has beenremoved from the system, a cylinder 66 of liquefied SF can be connectedto the. branch 56 (FIG. 5) with the interposition of a compressor 68.Delivery from the cylinder is in the gas phase, any impurities in theliquid SF not entering the system. When the SP gas has expelled all thenitrogen, the drain cock 52 is closed (preferably, the drain cock 52 isplaced very near the filling branch 56 to obtain a thorough purging, asshown in FIG. 4) and the compressor 68 presses the system to therequired value.

In the filling procedure shown in FIG. 6, the cylinder 66 of SP is justconnected to the branch 56 via a line 70 comprising a safety valve 72,and the cylinder 66 is heated; for instance, in awaterbath 74. As in theprocedure shown inFIG. 5, filling is by distillation, the systemtherefore being filled with purified dielectric.

In the procedure shown in FIG. 7, the system can be filled.

directly with liquid SF contained in a reservoir 76 similar to ahydraulic accumulator. A free piston 78 divides the reservoir 76 into afirst compartment 80-which is filled with liquid SP and which can beconnected by a duct 82 to thefilling branch 56-and a second compartment84 which can be connected to a source of pressurized liquid, such asoil. The source can comprise, for instance, a pump which draws oil froma reservoir 88 and delivers the oil at pressure to the compartment 84.Should the pressure become excessiveand when the piston 78 has reachedthe end of its travel, the pressurized oil discharges through a safetyvalve90. A cock 92 of the reservoir 76 is then closed and a cock 94 of aduct 96 is opened to allow the oil in the compartment 84 to return tothe tank 88. This procedure is useful for topping up the system, usingjust a small capacity reservoir 76.

A circuit breaker according to the invention can have provision forvisual indication of the amount of liquefied dielectric fluid present inthe system, for liquid SF looks-like water and is fully visible in aconventional sight glass type of water gauge. One such system is shownin FIG. 8, only an accumulator 28, of the kind shown in FIG 2,beingshown with the two fluid flow ducts 44, 24' connected to it. Ofcourse, the same system can be used for an accumulator to which only onepressurizing duct, such as the duct 24 in FIG. 1, is connected. Theaccumulator contains a volume 26 of liquid SF and thereabove a cushion32 of compressed gas, for instance, nitrogen, which pressurizes theliquid SF directly and not via a piston. A sight glass 98 is connectedto the accumulator 28, preferably via two isolating cocks 100, 102. Avisual check is therefore provided on the quantity of liquid dielectricand the quantity of cushioning gas. The accumulator can be rechargedwith nitrogen via the valve 34.

In the foregoing, the use of sulfur hexafluoride, SF as a liquefieddielectric gas has been more particularly considered, but everythingsaid in the foregoing also applies of course to cases where thedielectric is some other liquefiable gas having satisfactory dielectricproperties, such as Freons," or a mixture of several gases. Forinstance, it may be advantageous in some cases to use mixtures of thiskind in order to reduce the freezing point of C. dielectric (about 50 Cin the case of SP for circuit breakers installed outdoors in very. coldcountries.

In the variant shown in FIG. 9, the rupture chamber 2 is filled withliquid SF depending upon whether or not a hermetic partition is providedbetween the chamber 2 and the insulated column 4, the same can be filledeither with liquid SF or with insulating or some other dielectric, suchas gaseous SF In this embodiment, the liquid SF is kept pressurized by acushion 106 of pressurized gas, such as nitrogen; instead of beingtrapped in an accumulator as in the previous cases, the cushion 106 isimmediately above the level 106 of the dielectric in a compartment 108disposed above the: rupture chamber 2. The compartment 108 has a sightglass 98and a safety valve 110.

This form of pressurization may cause difficulties in outdoor circuit.breakers eptperiencing very wide daily temperature variations, forunless the compartment 108 were relatively large, there would be largepressure variations. However, this simple solution is very useful forcircuit breakers disposed indoors or, even better, in undergroundstations where the temperatures are substantially constant and pressuresvary little.

Also provided are a pressure gauge 36 and a thermometer 112 which, incooperation with the sight glass 98, enable the actual quantity ofnitrogen contained in the system to be known in all circumstances.

In a circuit breaker in accordance with this variant, it might appearsimpler for the gas cushion 104 to be formed by gaseous SF above theliquid SF However, when the moving contact 6 moves away from the fixedcontact 12at breaking, there would then be a risk of cavitation at theplace previously occu-. pied by the moving contact; and so it would bedifferent to. quench the are. On the other hand, if the gas used forthe. cushion 104 has a higher liquefaction pressure than SP and is moreparticularly nitrogen, the pressure above the level 106, such pressurebeing above the vapor tension of the SP forces the liquid dielectric tofill the gap left by the moving contact 6 as the. same separates fromthe fixed contact 12; the arc is therefore really quenchedin the liquiddielectric with an effect similar to blowing, since unionized dielectricis supplied" during the break action.

Only a single schematic kind of circuit breaker is shown in thedrawings,but the invention is of course of use with widely differing kinds ofcircuit breakers having single or multiple rupture chambers.

The invention is not of course limited to the embodiments described andshown and can be varied in many ways withinthe knowledge of the engineerin the art to suit the particular uses intended and without departurefrom the scope of the invention.

Iclaim:

1. A circuit breaker comprising a circuit interrupting chambercontaining a liquefied dielectric gas, a fixed contact member disposedin said chamber so as to lie within said liquefied gas, a movablecontact member disposed in said chamber so as to be in the liquefied gasfor all of its positions relative to the fixed contact member, andpressurizing means for continuously maintaining the liquefied gas undera pressure higher than the instantaneous vapor pressure of the liquefieddielectric gas, whereby all of said liquified gas is constantlymaintained in its liquid state.

2. A circuit breaker as set forth in claim 1 wherein the said liquefieddielectric gas is sulfur hexafiuoride SF 3. A circuit breaker accordingto claim 1, in which said pressure is continuously maintained above thecritical pres.- sure of said liquefied gas.

4. A circuit breakeras set forth in claim 1, wherein the saidliquefieddielectric gas is a perhalogenated hydrocarbon or flugene.

5. A circuit breaker as set forth in claim 1, in which said circuitinterrupting chamber is hydraulically connected to the liquidcompartment of a hydraulic accumulator, defining said pressurizingmeans, the compartment being filled with liquefied dielectric. gas whichis kept liquid by the pressure applied by the resilient means of theaccumulator.

6. A circuit breaker asset forth in claim 5, wherein the hydraulicaccumulator is at earth potential and the hydraulic connection betweenthe circuit interrupting chamber and the: accumulator is an insulatedduct.

7. A circuit breaker as set forth in claim 5, wherein the resilientmeans for pressurizing the accumulator take the form" of a pressurizedgas contained in the second compartment of the accumulator, theliquefaction pressure of the. latter gasbeing considerably greater thanthe liquefaction pressure of the liquefied dielectric gas at equaltemperatures.

8. A circuit breaker as set forth in claim 5, wherein the hydrauliccommunication between the liquid compartment of the accumulator and thecircuit interrupting chamber includes a nonreturn valve inhibitingliquid flow from the chamber to the accumulator; and a draining and flowduct connects the top part of the circuit interrupting chamber to theliquid compartment of the accumulator and also has a nonreturn valveinhibiting flow from the accumulator to the circuit interruptingchamber, so that the fluid can flow in closed circuit but onlyunidirectionally.

9. A circuit breaker as set forth in claim 8, wherein the draining ductcomprises a changeover means for selectively connecting the circuitinterrupting chamber either-the normal case-to the liquid compartment ofthe accumulator or to atmosphere, when it is required to purge thechamber of gases other than the dielectric gas.

10. A circuit breaker as set forth in claim 1, wherein the circuitinterrupting chamber is disposed at the top of an insulated column withinsulating liquid, and wherein the circuit interrupting chamber and theinternal space of the column are filled with said liquefied dielectricgas, the circuit interrupting chamber and the inside space of the columnbeing intercommunicating.

11. A circuit breaker as set forth in claim 1, wherein said pressurizingmeans comprises, above the circuit interrupting chamber containing theliquid dielectric, a compartment filled with a compressed gas, such asnitrogen, whose liquefaction pressure is appreciably greater than theliquefaction pressure of the liquefied dielectric gas at equaltemperatures, the compressed gas forming a resilient cushion forpressurizing the liquefied dielectric gas contained in the circuitinterrupting chamber.

1. A circuit breaker comprising a circuit interrupting chamber containing a liquefied dielectric gas, a fixed contact member disposed in said chamber so as to lie within said liquefied gas, a movable contact member disposed in said chamber so as to be in the liquefied gas for all of its positions relative to the fixed contact member, and pressurizing means for continuously maintaining the liquefied gas under a pressure higher than the instantaneous vapor pressure of the liquefied dielectric gas, whereby all of said liquified gas is constantly maintained in its liquid state.
 2. A circuit breaker as set forth in claim 1 wherein the said liquefied dielectric gas is sulfur hexafluoride SF6.
 3. A circuit breaker according to claim 1, in which said pressure is continuously maintained above the critical pressure of said liquefied gas.
 4. A circuit breaker as set forth in claim 1, wherein the said liquefied dielectric gas is a perhalogenated hydrocarbon or flugene.
 5. A circuit breaker as set forth in claim 1, in which said circuit interrupting chamber is hydraulically connected to the liquid compartment of a hydraulic accumulator, defining said pressurizing means, the compartment being filled with liquefied dielectric gas which is kept liquid by the pressure applied by the resilient means of the accumulator.
 6. A circuit breaker as set forth in claim 5, wherein the hydraulic accumulator is at earth potential and the hydraulic connection between the circuit interrupting chamber and the accumulator is an insulated duct.
 7. A circuit breaker as set forth in claim 5, wherein the resilient means for pressurizing the accumulator take the form of a pressurized gas contained in the second compartment of the accumulator, the liquefaction pressure of the latter gas being considerably greater than the liquefaction pressure of the liquefied dielectric gas at equal temperatures.
 8. A circuit breaker as set forth in claim 5, wherein the hydraulic communication between the liquid compartment of the accumulator and the circuit interrupting chamber includes a nonreturn valve inhibiting liquid flow from the chamber to the accumulator; and a draining and flow duct connects the top part of the circuit interrupting chamber to the liquid compartment of the accumulator and also has a nonreturn valve inhibiting flow from the accumulator to the circuit interrupting chamber, so that the fluid can flow in closed circuit but only unidirectionally.
 9. A circuit breaker as set forth in claim 8, wherein the draining duct comprises a changeover means for selectively connecting the circuit interrupting chamber either- the normal case- to the liquid compartment of the accumulator or to atmosphere, when it is required to purge the chamber of gases other than the dielectric gas.
 10. A circuit breaker as set forth in claim 1, wherein the circuit interrupting chamber is disposed at the top of an insulated column with insulating liquid, and wherein the circuit interrupting chamber and the internal space of the column are filled with said liquefied dielectric gas, the circuit interrupting chamber and the inside space of the column being intercommunicating.
 11. A circuit breaker as set forth in claim 1, wherein said pressurizing means comprises, above the circuit inTerrupting chamber containing the liquid dielectric, a compartment filled with a compressed gas, such as nitrogen, whose liquefaction pressure is appreciably greater than the liquefaction pressure of the liquefied dielectric gas at equal temperatures, the compressed gas forming a resilient cushion for pressurizing the liquefied dielectric gas contained in the circuit interrupting chamber. 